1. Field of the Invention
The present invention relates to an image sensing apparatus, an image sensing system, and a focus detection method and, more particularly, to an image sensing apparatus such as a digital still camera, a video camera and a silver-halide camera, an image sensing system, and a focus detection method.
2. Description of the Related Art
A contrast detection method and a phase-difference detection method are used as general methods that employ light beams that have passed through a photographing lens in focus detection and the adjustment of an image sensing device. The contrast detection method is often used in video cameras and digital still cameras, in which an image sensor is used as a focus detection sensor. This method pays attention to the output signal of the image sensor, in particular, information on a high-frequency component (contrast information), and information on the position of the photographing lens (where the evaluation value reaches a maximum) is set as an in-focus position. However, the contrast detection method, also called a hill-climbing detection method, is not suitable for high-speed focus adjustment operations because it is necessary to find an evaluation value while slightly moving the photographing lens, and to keep the photographing lens moving until the evaluation value is deemed to have reached a maximum.
The phase-difference detection method, on the other hand, is often used in single lens reflex cameras that use a silver-halide film, and is the technique that has contributed most to the practical use of auto focus (AF) single lens reflex cameras. According to the phase-difference detection method, a light beam that has passed through the exit pupil of the photographing lens is divided into two, and the two divided light beams are respectively received by a pair of focus detection sensors. The amount of defocus for the photographing lens in the focusing direction is directly determined by detecting the difference between the signals output according to the amount of light received, or in other words, an amount of relative positional shift in the direction in which the light beam is divided. Accordingly, once an accumulation operation is executed by the focus detection sensors, the amount of defocus and the direction of defocus can be obtained, making it possible to perform a high-speed focus adjustment operation.
In order to obtain signals corresponding to each of two divided light beams, generally, an optical path dividing mechanism, such as a half-mirror or reflecting mirror, having a quick return mechanism is provided in the image sensing optical path, and a focus detection optical system and an AF sensor are provided beyond the optical path dividing mechanism.
However, in order to perform focus detection using the phase-difference detection method, it is necessary to set the focus detection optical system such that the two divided light beams will not be vignetted even when a change in the exit pupil or image height of the photographing lens, a change in the zoom position, or the like, occurs. In particular, in a camera system to which a plurality of photographing lenses that have different exit pupil positions, diameters and degrees of vignetting can be attached, many restrictions are required to perform focus detection without vignetting. Consequently, there are problems such as not being able to set a wide focus detection region, not being able to improve the accuracy of two divided light beams by base-line length extension, and so on.
In view of the above circumstances, Japanese Patent Laid-Open No. 03-214133 discloses an example in which focus detection is performed after correcting a reduction in the amount of light to be converted into a focus detection signal caused by vignetting of a focus detection light beam based on information on the exit pupil of the photographing lens or the like. According to this example, focus detection using the phase-difference method can be performed even when slight vignetting exists, so wide range focus detection and highly accurate focus detection by base-line length extension can be performed.
In recent years, a technique has also been disclosed in which a phase difference detection function is provided for an image sensor so as to eliminate the need for a dedicated AF sensor and in order to achieve high-speed phase difference AF.
For example, according to Japanese Patent Laid-Open No. 2000-156823, a pupil-dividing function is provided for some of the light-receiving elements (pixels) of an image sensor by offsetting the sensitivity region of the light-receiving portion from the optical axis of the on-chip microlens. These pixels are used as focus detection pixels, and arranged with a prescribed spacing between image sensing pixel groups to perform phase-difference focus detection. The portions where the focus detection pixels are arranged correspond to portions where image sensing pixels do not exist, so image information is generated through interpolation using information from the peripheral image sensing pixels. With this example, focus detection using the phase-difference method can be performed at the image sensing surface, achieving high-speed and highly accurate focus detection.
However, Japanese Patents Laid-Open Nos. 03-214133 and 2000-156823 have the following problems.
According to Japanese Patent Laid-Open No. 03-214133, the reduction in the amount of light to be converted into a focus detection signal caused by vignetting of the photographing lens is corrected based on the information on the photographing lens side. However, the degree of vignetting varies according to not only the information on the photographing lens side, but also according to a production error on the camera side. In particular, in a single lens reflex camera, because a complicated conversion and separation of the optical path are performed by a focus detection optical system, the production error caused thereby is large. Accordingly, even when the reduction in the amount of light is corrected based on only the information on the photographing lens side, an error occurs in the result of the focus detection.
According to Japanese Patent Laid-Open No. 2000-156823, because pupil division is realized using the relative positional relationship between the on-chip microlens and the light-receiving portion, a problem arises in that the pupil division undergoes a large shift due to a production error in the on-chip microlens. In such pupil division using an on-chip microlens, the light-receiving portion located several microns below the microlens is reversely projected to the exit pupil position of the photographing lens that is located several tens to several hundreds of millimeters away and, as a result, the imaging magnification becomes very large. Accordingly, a slight production error in the on-chip microlens can result in a large shift, causing significant vignetting in the focus detection light beam, and making focus detection not possible.